Intermodulation interference (IMI) occurrs when high frequency (HF) transmitters and receivers are placed in close proximity, particularly on ships and aircraft, due to non-linear conduction through metal-oxide-metal (MOM) junctions in the supporting structure. This is also known as "rusty bolt" intermodulation interference. IMI is a problem where there are any non-welded metal-to-metal joints but pose particular problems on ships due to MOM junctions in or at life lines, ladders, flag poles, bond straps, boat davits, stays, nets, chains, etc. Steel, aluminum, brass and copper are examples of common metals that form MOM Junctions but any metal that can form an oxide can form a MOM junction. MOM junctions interfere with signal transmission throughout the HF to infrared range, but pose particular problems for communications in the 2-30 MHz region and for VHF frequencies.
The electrical behavior of the MOM junction responsible for the IMI is similar to that of two parallel back-to-back semiconductor diodes, each operating in its forward conducting region. The non-linear conduction of the MOM junction or the analagous diode configuration is the mixing mechanism that generates IMI. This mixing action, and thus the IMI, can be suppressed if the current through the MOM junction is by-passed through a linear conductive path, i.e., one with low resistance or low reactance.
Eliminating or reducing IMI by providing a linear conductive path has traditionally been accomplished by clamping or welding conductive ground straps across the MOM junction thus shorting-out the MOM junction or by replacing the MOM junction with non-conductive material thus eleminating the MOM junction. These solutions, although effective, have limitations. Using non-conductive materials is prohibitive where structural strength or electrical conductivity is requied. Welding or clamping straps cannot be used where there is a multiplicity of MOM junctions, such as in chains or twisted cable, or where dissimilar metals are involved. Additionally, the use of non-conductive materials or conductive straps is very expensive. To overcome these limitations while effectively reducing IMI, an inexpensive method that maintains the structural integrity of supporting hardware while providing a highly conductive current path is required.
Previous methods used to overcome IMI relied upon "rust removal compositions" which have incorporated the general principles of chelating the ferric ion, incorporating the ion into a polymer material, and removing the "rust" with the polymer material. Barabas, U.S. Pat. No. 4,424,079, used a vinylpyrrolidone/maleic acid copolymer to remove rust by this general technique. Panayappan, in U.S. Pat. No. 4,325,744, used a vinylpyrrolidone/ethylenediaminetetraacetic acid (EDTA) or similar chelating agent to encapsulate rust which can be peeled from the surface and disposed of as a solid waste. Neither of these references, however, contemplate changing the oxidation state of the metal to increase conductivity nor consider the application of the composition to nonferric oxides nor did they consider incorporation of a chemical reducing agent that would prevent further corrosion and IMI generation. Thus, the usefulness of the techniques is limited to "rust removal" processes involving ferric ions. The references do not consider increasing structural intergity or providing more conductive paths necessary to reduce IMI, applications to other metals, or inhibition of further corrosion.
A method is, therefore, needed which can eliminate IMI by providing a low resistance and low reactance linear conductive current path through potential MOM junctions.